Method and apparatus for braking and stopping vehicles having an electric drive

ABSTRACT

A method and apparatus are provided for braking and stopping a vehicle whose powertrain includes an electric drive. The electric drive is used to generate braking torque which is used to decelerate the vehicle down to a full stop. The braking torque is achieved using any of several closed loop speed control systems. The system can be used as a substitute for or as a supplement to conventional friction bakes.

FIELD OF THE INVENTION

This invention generally relates to vehicles with electric drivesystems, and deals more particularly with a method and apparatus forbraking and stopping the vehicle using the electric drive.

BACKGROUND OF THE INVENTION

Many recent designs of electric powered and hybrid electric poweredvehicles employ a regenerative braking system in order to increaseoperating efficiency. During a braking event, the electric motor whichnormally drives one or more traction wheels is switched to operate as anelectrical generator. Using the momentum and kinetic energy of thevehicle, the electric drive motor generates electricity that may be usedto recharge on-board energy storage systems, such as batteries and ultracapacitors, power accessories, or power auxiliary on-board systems.

Regenerative braking systems are particularly effective in recoveringenergy during city driving, where driving patterns of repeatedacceleration and decelerations are common. Electric drive vehiclesemploying regenerate braking typically utilize traditional frictionbrakes, along with a vehicle control system that coordinates theoperation of the friction brakes and the regenerative brake in order toprovide adequate stopping ability while making dual brake operationsessentially transparent to the driver. Normally, such a control systemcontrols the electric motor torque to perform regenerative braking untilthe vehicle decelerates to a certain speed at which time the frictionbrakes are gradually applied to bring the vehicle to a compete stop.

The dual braking strategy described above may not be optimum for certaintypes of electric drive configurations, and may not be appropriate forconfigurations where it is desirable to completely avoid frictionbraking components. For example, a two axle vehicle might be providedwith friction brakes on the wheels of only one axle; clearly it would bedesirable to provide an electric means of fully braking the axle notequipped with friction brakes. In some configurations, it may bedesirable to completely avoid the use of friction brakes, thusnecessitating the use of some electronic means of achieving adequatebraking. Even in those configurations where all wheels are equipped withfriction brakes, it may be desirable to provide frictionless electricbraking for each axle in the event that the friction brakes areintentionally or unintentionally disabled for any reason.

Accordingly, a need exists in the art for a braking system for vehicleswith electric drive systems capable of providing frictionlessdeceleration and braking of the vehicle to all speeds down to andincluding zero speed, regardless of the configuration of the vehicle'smotor drive, axles and wheels. The present invention is intended tosatisfy this need.

SUMMARY OF THE INVENTION

A system is provided for decelerating and stopping a vehicle equippedwith an electric drive system without the need for friction brakes, orwith reduced need for friction brakes on at least one wheel. Brakingdeceleration of the vehicle is achieved by controlling the electricdrive motor to produce negative torque which is transmitted to thewheels, enabling deceleration down to and including zero speed. Tomaintain the stopping position of the vehicle on grade inclines, theelectric drive motor is controlled to produce a small, compensatingamount of positive or negative torque at zero speed, depending on thedirection of the incline. The system may also be used as a back-upbraking system for vehicles equipped with friction brakes, or to providesupplemental braking on axle assemblies that are not equipped withfriction brakes.

One advantage of the invention is that the braking system can be usedwith reduced need for conventional friction brakes. Another advantagelies in the ability of the present braking system to decelerate thevehicle down to and including zero speed, and maintain the vehicle at acomplete stop under various driving conditions, such as on a grade,using the speed control loop of the electric drive. A still furtheradvantage of the invention is that the need for conventional frictionbrakes may be completely avoided.

In accordance with a first embodiment of the invention, a method isprovided for braking and stopping a vehicle having at least one tractionwheel driven by an electric motor. Braking and stopping is achieved bysensing a speed parameter related to the speed of the vehicle, sensing acommanded braking rate, generating a motor control signal using thesensed speed parameter and commanded braking rate, producing a negativetorque using the electrical drive motor, applying braking forces to thetraction wheels using the negative torque, and controlling the amount ofnegative torque produced by the electric drive motor using the motorcontrol signal to achieve the commanded braking rate. The motor controlsignal may include a torque command signal, a speed command signal or acombination of these two signals. The torque command signal can be usedto control the motor until the vehicle decelerates to a pre-selectedspeed, following which a speed control signal is used for motor control.The motor control signal is based on torque commands determined by theposition of the vehicle's brake and accelerator pedals. The sensed speedparameter may include either the speed of the drive motor, the speed ofat least one wheel of the vehicle, or a combination of these sensedspeeds.

In accordance with a second embodiment of the invention, a system isprovided for braking and stopping a vehicle powered by an electric motordriving at least one traction wheel. The system includes a closed loopspeed control loop whose speed command is a zero speed signal. Thisclosed loop system features modification of its control signal (torquecommand signal for the electric drive) by a bipolar torque limitsignal-pair. The limit signal-pair is directly derived from a torquecommand that is obtained by the vehicle system controller, with theaccelerator and brake pedals as inputs. The torque command of thevehicle system controller may be used for driving and deceleration athigher speeds, but the torque-limited speed control loop is used forbringing the vehicle to a stop.

These non-limiting features, as well as other advantages of the presentinvention may be better understood by considering the following detailsof a description of a preferred embodiment of the present invention. Inthe course of this description, reference will frequently be made to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary block diagram of an electric drive system for avehicle;

FIG. 2 is a graph showing the brake torque as a function of speed,produced in a vehicle equipped with the combination of electric andfriction braking systems;

FIG. 3 is a graph showing commanded torque and actual electric drivebraking torque as a function of speed, in a vehicle equipped with theelectric braking system, according to one embodiment of the presentinvention;

FIG. 4 is a graph of the actual electric drive brake torque as afunction of speed generated in accordance with another embodiment of thepresent invention;

FIG. 5 is a graph showing actual electric drive brake torque as afunction of speed, produced according to a further embodiment of thepresent invention;

FIG. 6 is a block diagram of a system for braking and stopping anelectric drive vehicle, which includes torque limiting with bipolarsignals and a speed control loop, in accordance with the presentinvention;

FIG. 7 is a block diagram of a system for obtaining averaged motorspeed; and,

FIG. 8 is a block diagram of a speed control loop having a nested torquecontrol system, used in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention relates to a method and apparatus for decelerating,braking and stopping a vehicle equipped with an electric drive systemwhich includes an electric motor. A typical electric drive system 10 isshown in FIG. 1. An electric motor 12 mounted on the vehicle's chassishas an output drive shaft 14 which is connected through a differentialgear-set 16 to a drive axle 18 carrying one or more traction wheels 20.Energy for powering the motor 12 is derived from an on-board storagebattery 22 which provides DC power that is converted by an inverter 24into AC power used to drive the motor 12. Although an AC motor 12 hasbeen disclosed here, it should be noted that the present invention issuitable for use with a variety of DC and poly-phased AC motors. Avehicle control system 26 coordinates and controls the operation of theenergy storage and drive components, and manages system functions suchas charging, engine starting and stopping and regenerative braking. Thevehicle control system 26 may implement any of a variety of knowncontrol strategies, using software programs and input informationderived from a variety of on-board sensors 28, as well as acceleratorpedal and brake pedal position information 30. It should be noted herethat although a drive system 10 has been shown employing only a singlemotor 12, the present invention may be used in drive systems employingmultiple electric motors, alternate fuel sources and hybridconfigurations employing at least one drive electric motor. Furthermore,the motor 12 may be in the form of a wheel motor that is incorporateddirectly into one or more wheels on the vehicle. For sake of conveniencein the describing and claiming the invention, “negative torque” appliedto a drive wheel shall mean a torque that opposes the motion of thevehicle, whereas a positive torque applied to the wheel shall mean atorque that favors the vehicle's motion.

The vehicle control system 26 may deliver either a torque command or aspeed command to the motor 12, having a polarity and magnitude that isbased on the positions of the accelerator pedal and the brake pedal 30.The torque command can be either positive or negative in both drive andreverse “gear” selected as the desired direction of travel; as is knownin the art, a positive command results in traction torque while anegative command results in braking or deceleration torque. The detailsof generating both torque and speed commands as a function of pedalpositions depend on the particular vehicle configuration and will bebased on any of various control strategies which are well known in theart. A torque or speed command developed by the control system 26 isdelivered to the inverter 24, causing the motor 12 to produce positivetorque which is delivered by a driven axle 18 to traction wheels 20.Based on the position of the accelerator and brake pedals 30, thecontrol system 26 switches the motor 12 to its regenerative mode inwhich the motor 12 acts as an electrical generator, converting thevehicle's kinetic energy into electrical energy used to recharge thebattery 22. During regenerative braking, motor 12 produces a negativetorque.

The relationship between the negative torque produced by motor 12 andthat produced by the vehicle's friction brakes is better understood byreference to FIG. 2 which plots torque of the motor 12 as well asfriction brake torque as a function of vehicle speed. The plot of FIG. 2corresponds to a typical vehicle that employs friction brakes on atleast one wheel, in addition to regenerative braking provided by atleast one electric drive motor on the vehicle. Different modes ofbraking torque occur over three distinct regions respectively designatedas Region 1, Region 2 and Region 3. At higher vehicle speeds shown inRegion 1, regenerative braking results in an electric drive torquecommand 32 which continues until the vehicle brakes to a speed at whichfriction brakes are applied to produce friction brake torque 34 at thebeginning of Region 2. As the friction brake torque 34 increases, theelectric drive torque command 32 ramps down until Region 3 is reachedwhere the braking torque is entirely the result of the friction brakes.In Region 3, friction brake torque 34 reaches a constant value and theelectric drive torque produced by the torque command 32 is zero.

In accordance with the present invention, deceleration of the vehicledown to and including zero speed (a complete stop) is accomplished usingnegative torque produced by the motor 12, without the use of brakingtorque supplied by friction brakes.

In accordance with one technique of the present invention, the vehiclecontrol system 26 delivers signals to the motor 12 commanding negativetorque 32 as shown in FIG. 3, down to a pre-selected speed where thenegative torque is then ramped down to zero. The actual electric drivetorque produced by the commanded torque signal 32 is designated by thenumeral 36 and can be seen to closely follow the commanded torque curve32. Thus, using this first technique, only torque control is used fordecelerating and stopping the vehicle. This technique is suitable forvehicle operation on essentially level ground. If there is change inground elevation or grade resulting in an upward slope or downward slopethere could be some movement of the vehicle after coming to a near stop.Thereafter, the system will react to the vehicle's speed and effectdeceleration but perfect holding at zero speed may not be achieved withthis technique if material ground (elevation or grade) slope is present.Accordingly, it may be necessary in using this technique to apply thevehicle's parking brakes, either manually or automatically through theelectronic controls, in order to assure that the vehicle is held in astationary position.

In accordance with another technique, the motor 12 is used to producenegative torque down to a pre-selected speed using the torque controlmode previously described, following which motor 12 is switched to speedcontrol mode in which the speed command is zero or another commanddetermined by the accelerator and brake pedal position inputs 30. FIG. 4is a graph of torque versus speed, which illustrates the secondtechnique more clearly. As can be seen at higher speeds, using thetorque control mode 56, the electric drive actual torque 52 isrelatively constant down to a pre-selected speed where the controlscheme is switched over to speed control mode 58. Speed control resultsin the actual drive torque 52 ramping down from a corner point 54 tozero speed where the vehicle reaches a full stop. This techniqueprovides adequate position holding when the vehicle stops on a (groundelevation/grade change) material grade slope, since at near zero speed,a speed control loop used to implement the technique generates enoughtorque to compensate for the slope.

If desired, the motor 12 can be operated in a speed control modethroughout the deceleration and stopping procedure using a zero speedcommand or other speed command that is based on the position of thepedals 30. FIG. 5 is a graph showing the actual electric drive torque 60during the deceleration and stopping procedure performed using only thespeed control mode of operation. It can be seen that the plot of theactual negative torque 60 is more gradual in the reduction of torque asspeed decreases. Moreover, it can be seen that the actual torque 60becomes slightly positive at zero speed. This slightly positive torqueat zero speed corresponds to a situation where the vehicle is on aslightly positive or upward grade incline. The slight amount of residualpositive torque maintains the vehicle in its stopped position, andcompensates for the incline. Similarly, if the vehicle comes to rest ona downward grade incline, a small amount of negative torque is appliedat zero speed in order to maintain the position of the vehicle on theincline.

In some applications and vehicle configurations it may not be convenientto translate accelerator and brake pedal inputs 30 into a speed command.In order to address this possibility, a further technique is provided inaccordance with the present invention which is illustrated in the blockdiagram of FIG. 6. Accelerator and brake pedal positions 62, 64 aretranslated into torque commands by a torque command generator 66; thesetorque commands may be either negative or positive, depending upon thevehicle's operating conditions. The torque commands are translated intoa bi-polar signal (two states—state 1 or state 2) by a bi-polar signalgenerator 68 which is used as a bi-polar torque limiter, with eitherpositive or negative limit values 70, 72, to further control the torquecommand signal 74 that is used to control the electric drive 10.

The speed control loop includes a dynamic compensator 38 which outputs atorque command signal 74 to the electric drive 10 after being subjectedto limits 70, 72. The electric drive produces a torque 50 and motorspeed 48. The motor speed 48 is fed back in a feedback loop 46 where itis compared at 40 with the motor speed command (normally zero speed) andthe error information is fed to the dynamic compensator 38. The outputof the dynamic compensator 38 is the torque command signal 74 which issubjected to the limits 70, 72 and hence may become limited. Theresulting torque command signal is the final torque command signal forthe electric drive 10. One function of the speed control loop is togenerate electric drive torque command whose function is to reduce thespeed of the motor 12 to zero by closed loop control action. Aspreviously noted, due to the action of the speed control loop, thetorque at zero and near zero speeds will be positive (corresponding tothe traction) if there is a grade opposing the forward motion of thevehicle, and it will be negative if there is a grade favoring theforward motion of the vehicle. As can be seen in FIG. 6, the torquecontrol loop is nested within the speed control loop with the motorspeed 48 being fed back in loop 46 to be combined with the commandedmotor speed. The commanded motor speed is the desired vehicle speedmultiplied by an appropriate gear ratio related to the gear-set 16 (FIG.1). The initial condition of the dynamic compensator 38 should be set tothe value of the torque command value existing at the moment precedingthe transition from torque control to speed control. It should be notedhere that if the brake pedal is not depressed a sufficient amount, thetorque limiting which is imposed will be small in magnitude and theelectric drive torque produced may not be sufficient to obtain zeromotor speed and vehicle speed. In this case, the vehicle operator willdepress the brake pedal a further amount in order to obtain zero speed.In a similar manner, if the vehicle is being brought to a stop on asteep slope, the operator may need to depress the brake sufficientlyand, possibly fully, in order to achieve and/or maintain a completevehicle stop. When the torque command as output by the pedal interpreterchanges sign, the speed control loop is exited. It should be appreciatedhere that if the driver does not depress the brake sufficiently and theabove described torque limiting function is not employed, the vehiclecan be held on a grade solely through the use of the speed controlfunction. In other words, if the torque command signal 74 of the closedloop system of FIG. 6 is not subjected to the limits 70, 72, then thesystem would be able to achieve and maintain zero speed on a grade eventhough the brake is not depressed sufficiently.

It should be further noted that in each of the control techniquesdescribed above, regeneration cannot take place at low speeds, eventhough the sign of the electric drive torque is negative. Due to certainfixed losses in the drive system, the battery will be supplying power atspeeds near zero, even though the generated torque is negative.Moreover, due to the action of the speed control loop, the torque atzero and near zero speeds will be positive (corresponding to traction)if there is a grade opposing the forward motion of the vehicle, and itwill be negative if there is a grade favoring the forward motion of thevehicle.

Depending upon the vehicle and electric drive configuration, some smallinaccuracies of the motor speed signal may occur at vehicle speeds nearzero. This may be caused in part by noise and quantization effects dueto the operation of motor speed encoders. Thus, it may be desirable toimprove the motor speed detection in certain applications, and in thisconnection a technique is shown in FIG. 7 for improving speed detectionaccuracy. A plurality of wheel speed sensors, WSS#1-WSS#4 are used incombination with a motor speed sensor 84 to arrive at a speed signalused for the control process. The wheel speed sensor information iscombined and averaged at 76 and multiplied by a scale factor at 78 whichis related to the gear ratio between the motor 12 and wheels 20. Theaveraged and scaled wheel speed information is added to the motor speed84 at 80 and then multiplied by a factor of ⅕ at block 82. The resultingmotor speed value having superior accuracy is used at the feedbacksignal and loop 46 (FIG. 6).

FIG. 8 shows a simplified speed control loop with nested torque controlsystem. In this embodiment, the torque control loop is a loop within thespeed control loop, and the motor speed is measured and used as thefeedback signal. Commanded motor speed is the desired vehicle speedmultiplied by the appropriate gear ratio. The same scheme given aboveand shown in FIG. 7 can be used for obtaining motor speed in thoseembodiments of the invention wherein the speed control loop shown inFIG. 8 is used.

From the foregoing description it is apparent that a novel method isprovided of braking and stopping a vehicle having at least one tractionwheel. The method includes the steps of sensing a speed parameterrelated to the speed of the vehicle, sensing a commanded brake rate,generating a motor control signal using the sensed speed parameter andcommanded braking rate, producing a negative torque using the electricmotor, applying a braking force to the traction wheel using the negativetorque, and controlling the amount of negative torque produced by theelectric motor using the motor control, signals to achieve the commandedbraking rate. The motor control signal may be a power command signal ora force command signal.

It is to be understood that the specific methods and techniques whichhave been described are merely illustrative of one application of theprinciples of the invention. For example, if the motor torque capabilityis limited, the present method can be utilized in combination withfriction brakes that may or may not be downsized. Moreover, the presentinvention does not require the elimination of friction brakes on atleast one wheel. Numerous modifications may be made to the method andsystem as described without departing from the true spirit and scope ofthe invention.

1. A method of braking and stopping a vehicle having at least onetraction wheel driven by an electric motor, comprising the steps of:sensing a speed parameter related to the speed of the vehicle; sensing acommanded braking rate; generating a motor control signal to achieve adesired braking action; producing a negative torque using the electricmotor; applying a braking force to the traction wheel using the negativetorque produced by the electric motor; and, controlling the amount ofnegative torque produced by the electric motor using the motor controlsignal to achieve the commanded braking rate.
 2. The method of claim 1,wherein the motor control signal is a torque command signal.
 3. Themethod of claim 1, wherein, the motor control signal is a speed commandsignal
 4. The method of claim 1, wherein, the motor control signal is apower command signal.
 5. The method of claim 1, wherein, the motorcontrol signal is a force command signal.
 6. The method of claim 1,wherein the motor control signal is generated using the sensed speedparameter and the commanded braking rate.
 7. The method of claim 1,wherein the step of applying braking force is continued until thevehicle is stopped.
 8. The method of claim 1, wherein the step ofgenerating the motor control signal comprises: producing a torquecommand signal used to control the motor until the vehicle deceleratesto a preselected speed, and producing a speed control signal used tocontrol the motor after the vehicle has decelerated to the preselectedspeed.
 9. The method of claim 1, wherein the step of applying brakingforce is continued until the vehicle decelerates to a stop.
 10. Themethod of claim 1, wherein the step of generating the motor controlsignal comprises producing a torque control signal based on thepositions of the vehicle's brake and accelerator pedals.
 11. The methodof claim 1, wherein the step of generating the motor control signalcomprises: generating a negative torque control signal used to controlthe amount of negative torque produced by the electric motor until thesensed speed parameter reaches a preselected value, and then, generatinga speed control signal used to control the speed of the electric motorafter the sensed speed reaches the preselected value.
 12. The method ofclaim 1, wherein the step of sensing the speed parameter comprises:sensing the speed of the motor, and sensing the speed of at least onewheel of the vehicle.
 13. The method of claim 1, wherein the step ofsensing the commanded braking rate comprises sensing a parameter relatedto the operation of the brake pedal.
 14. The method of claim 1, whereinthe step of generating the motor control signal comprises: generating atorque command signal, generating a speed command signal, modifying thespeed command signal using the torque command signal.
 15. The method ofclaim 14, wherein; the step of sensing the speed parameter comprisesgenerating a motor speed signal representing the speed of the electricmotor, and the step of generating the motor control signal comprisesfeeding back the motor speed signal in a motor control feedback loop,and controlling the motor using the feedback signal and the modifiedspeed command signal.
 16. A method of braking and stopping a vehiclepowered by an electric motor driving at least one traction wheel,comprising the steps of: sensing a speed parameter related to the speedof the vehicle; sensing a commanded braking rate; generating a commandedmotor speed signal; generating a torque limiting signal using thecommanded braking rate; generating a torque command signal using thecommanded motor speed signal and torque limiting signal; controlling themotor using the torque command signal to produce negative torque; and,applying a braking force to the traction wheel using the negativetorque.
 17. The method of claim 16, wherein the step of sensing thespeed parameter comprises sensing the speed of the motor.
 18. A methodof claim 16, wherein the step of sensing the speed parameter comprisessensing the speed of at least one wheel of the vehicle.
 19. The methodof claim 16, wherein the step of sensing the commanded braking ratecomprises sensing a parameter related to the operation of a brake pedalon the vehicle.
 20. The method of claim 16, wherein the step ofgenerating the torque command signal comprises using the sensed speedparameter as a feedback signal in a closed feedback loop, and performingdynamic compensation of the torque command signal using the feedbacksignal.
 21. The method of claim 16, wherein the step of generating thetorque command signal comprises modifying the torque command signalusing the torque limiting signal.
 22. The method of claim 16, whereinthe torque limiting signal is determined by at least one of the brakepedal or accelerator pedal inputs.
 23. The method of claim 16, furthercomprising the steps of: controlling the motor using the torque commandsignal to produce positive torque; and, applying a braking force to thetraction wheel using the positive torque.
 24. A system for braking andstopping a vehicle powered by an electric motor driving at least onetraction wheel, comprising: a torque command signal generator forgenerating a torque command signal used to drive the motor and producenegative torque for braking the traction wheel; and, a torque limitingsignal generator for converting brake pedal and accelerator pedalposition values into torque limiting signals used to modify the torquecommand signal.
 25. The system of claim 24, wherein the torque commandsignal generator comprises a dynamic compensator for compensating forthe effects of changes in motor speed.
 26. The system of claim 24,wherein the torque command signal generator comprises a closed speedcontrol loop.
 27. The system of claim 26, wherein the torque limitingsignal generator includes a bipolar signal generator.